Systems of this kind are known from various prior art publications. Typical examples are described in the European patent application EP-0,295,942 and the U.S. Pat. No. 3,932,264.
During operation a sample of the solution to be analyzed is injected by the input device into the input opening of the capillary tube. In the following migration process the voltage source is switched on to start the migration in the capillary tube in the course of which each ion will start to move according to its own mobility through the capillary tube. Because of the mobility differences the ions in the electrolyte will be separated into zones such that ions of different type will pass the detector at different times which are characteristic for the type of ions. The strength of the signal generated by the detector circuit as a zone of ions passes the detector is related to the concentration of the respective type of ions.
A problem in said prior art systems is the calibration of the system. If the length of the capillary tube between the input opening and the detector area is selected long enough, establishing a relatively long migration period, the separation between the various kinds of ions will be distinct enough to enable the identification of each ion type on the basis of the elapsed time period between the moment of sample injection and the moment the respective ions pass the detector area. Especially if the number of different types of ions is rather restricted the influence of variable parameters such as the electrolyte composition and the temperature thereof will be negligible and identification of the ion type will be rather straightforward and can be performed by a suitably programmed processor.
However, an accurate determination of the concentration of the ions of a specific type is not so easy. In the course of the migration through the capillary tube the ions are separated type by type in bands or zones which have a certain width, said width being proportional to the number of ions. Because of said width the signal generated by the detector circuit will gradually increase to a peak level and will from there decrease. Furthermore a "background" signal will be generated even if only the electrolyte without any injected ions is present in the detector area, the level of the background signal being dependent on the composition of the electrolyte. Further parameters influencing the generated signal are the temperature of the electrolyte in the capillary tube, and the degree of stabilization of the voltage supplied by the voltage source for developing the voltage gradient in the capillary tube.
Even if the influence of temperature and voltage stability is reduced to a negligible level still the calibration of the system is rather cumbersome and inaccurate, mainly based on comparison of the detector signals with signals received from standard samples containing well-known concentrations of a specific ion.